1. Field of the Invention
The present invention relates to processes for the recovery of crude tall oil from tall oil soaps and in particular to a process for the concurrent acidulation and extraction of fatty and resin acids from the soaps with supercritical carbon dioxide.
2. Brief Description of the Prior Art
Tall oil soaps are obtained from the black liquor resulting from the alkaline digestion of wood. When the black liquor is concentrated and allowed to settle, the tall oil soap rises as a top layer. The top layer is skimmed off and may be subsequently acidified with a strong polar acid, usually sulfuric acid, to convert the tall oil soaps to crude tall oil, i.e., free fatty and resin acids. The reaction may be represented by the schematic formulae: EQU 2R--COONa+H.sub.2 SO.sub.4 .fwdarw.2R--COOH+Na.sub.2 SO.sub.4 EQU tall oil soap+sulfuric acid.fwdarw.crude tall oil+sodium sulfate
when sulfuric acid is used as the acidulation agent.
The acidified soap skimmings are allowed to stand and separate into three layers--a crude tall oil layer containing the fatty acids, resin acids and neutrals, a lignin layer, and a sulfate-brine layer. The separated crude tall oil layer may be fractionally distilled to separate and purify the fatty acid and resin acid fractions. The sulfate-brine layer has limited commercial value and, when reintroduced into the pulp mill liquor system, can adversely affect the mill sulfur balance. Accordingly, it has been desirable to find a process for acidulation and recovery of fatty acids and resin acids from tall oil soaps, without the need to use sulfuric acid.
Prior art attempts at solving this problem have met varying degrees of success. For example, acidulation of tall oil soaps with carbon dioxide has been known; see U.S. Pat. No. 2,232,331. However, commercialization of this approach has been hampered because of the poor yields (resulting from the fact that sufficiently low pHs could not be achieved with carbon dioxide). Recently, it has been shown that yields from the carbon dioxide acidulation of tall oil soaps could be improved by doing so in the presence of a water immiscible solvent. Since the tall oil acids are preferentially soluble in the water-immiscible solvent, the equilibrium of the acidulation reaction is favorably shifted toward the acid products; for example see U.S. Pat. No. 4,075,188.
Processes for the recovery of carboxylic acids from aqueous solutions of their salts through conversion of the salts to free acids by acidulation, and extraction of the free acids with supercritical fluid carbon dioxide has also been described; see U.S. Pat. No. 4,250,331. However, in the latter process, less than about 20% of the salts are recovered as free acids. A fuller description of supercritical fluid extractions may be found in U.S. Pat. No. 3,969,196.
It has now been found that tall oil soaps can be converted to the free acids in near quantitative yields by acidulation with supercritical carbon dioxide, thereby avoiding use of sulfur-based acidulation chemicals. The fatty/resin acids are extracted by the supercritical carbon dioxide. A further advantage is found in that the extracted fatty/resin acids are free of lignin, fibers, inorganics and like contaminants generally associated with the tall oil soaps.